A soybean seed contains therein a large quantity of the enzyme lipoxyenase which accelerates oxidation of unsaturated fatty acids such as linolic and linolenic acids, which are components of soybean oil.
The action of the enzyme is so intensive that when the dry seed is crushed, and under conditions where the enzyme, soybean oil and oxygen (in air) are mixed and contacted with each other, oxidation occurs. The produced, oxidized substance (hydroperoxide), is further decomposed to produce an aldehyde or alcohol. These products have an undesirable taste characterized by a grassy flavor and bitterness, thus posing a significant problem in the production of soybean oil and soybean protein foods.
The normal soybean seed has, three kinds of lipoxigenase, L-1, L-2 and L-3. Their physical and chemical properties are as follows:
L-1:
(a) L-1 has an optimum pH which is around pH9, and it has a low activity for any substrate with a pH which is around neutral.
(b) The L-1 enzyme acts well on free fatty acids such as linolic acid, but it does not act well on neutral fats such as triglyceride.
(c) L-1 is relatively heat stable as compared to L-2 and L-3.
L-2:
(a) L-2 has an optimum pH which is around pH7.
(b) The L-2 enzyme has a much higher activity for arachidonic acid as compared to L-1 and L-3. It acts well on neutral fats such as triglyceride.
L-3:
(a) L-3 is similar to L-2. It has an optimum pH which is around pH7.
(b) The L-3 enzyme, relatively, acts well on free fatty acids such as linolic acid and neutral fats such as linolic acid ester or triglyceride.